You couldn't get any more apples and oranges if you tried. Go with Buffy if you are interested in fantasy and vampires. Go with Farscape if you are more interested in outer space and aliens.
The differences aren't that significant. Demons and aliens don't play fundamentally different roles in a dramatic sense, and magic tech isn't all that different from plain old magic when it comes down to it.
Both shows thrive on the strength of their characters and arcs, not on their particular set of conceits.